The fix for this bug has been committed: https://bugs.php.net/bug.php?id=73398 Description: ------------ I have found some vulnerable code at ```php_escape_html_entities_ex()``` function. ```php_escape_html_entities_ex()``` function creates a new zend_string object to store html data. The size of destination string depends on the size of source string. ( refer at `ext/standard/html.c:1272` ) ``` c PHPAPI zend_string *php_escape_html_entities_ex(unsigned char *old, size_t oldlen, int all, int flags, char *hint_charset, zend_bool double_encode) { ... /* initial estimate */ if (oldlen < 64) { maxlen = 128; } else { maxlen = 2 * oldlen; if (maxlen < oldlen) { zend_throw_error(NULL, "Input string is too long"); return NULL; } } replaced = zend_string_alloc(maxlen, 0); ... } ``` If `oldlen` is equal to PHP_INT_MAX, `maxlen` will be an unexpected value and `zend_string_alloc()` function will allocate a small memory range. Due to missing check of size before calling `zend_string_alloc()`, this new memory range can not use to store large html data and lead to heap overflow. I can overwrite other objects of PHP in memory. This bug is only triggered in 32bit machine. Solution: It should be `zend_string_alloc_safe` instead of `zend_string_alloc`. Test script: --------------- ``` php ``` Actual result: -------------- Open php program in gdb and run test script, set a breakpoint at line in file `ext/standard/html.c:1269`. When debugger stops, we have `oldlen=0x7fffffff`. Because `oldlen` is bigger than 0x64, `maxlen` is equal to twice `oldlen`. `maxlen` is equal to 0xfffffffe. ``` [----------------------------------registers-----------------------------------] EAX: 0xfffffffe EBX: 0x1 ECX: 0x10 EDX: 0x5 ESI: 0xb7814100 --> 0x2 EDI: 0xfffffffe EBP: 0xbfffbf68 --> 0xbfffbfb8 --> 0xbfffc084 --> 0x0 ESP: 0xbfffbee0 --> 0x80001000 ('A' ...) EIP: 0x826e37a (: call 0x82fc010 <_emalloc>) EFLAGS: 0x202 (carry parity adjust zero sign trap INTERRUPT direction overflow) [-------------------------------------code-------------------------------------] 0x826e371 : mov edi,DWORD PTR [ebp-0x34] 0x826e374 : lea ecx,[edi+0x14] 0x826e377 : and ecx,0xfffffffc => 0x826e37a : call 0x82fc010 <_emalloc> 0x826e37f : mov esi,eax 0x826e381 : mov DWORD PTR [eax],0x1 0x826e387 : mov DWORD PTR [eax+0x4],0x6 0x826e38e : mov DWORD PTR [eax+0x8],0x0 [------------------------------------stack-------------------------------------] 0000| 0xbfffbee0 --> 0x80001000 ('A' ...) 0004| 0xbfffbee4 --> 0xb7ce07e9 (: pop ebx) 0008| 0xbfffbee8 --> 0xb7ce07f7 (: add ecx,0xc7809) 0012| 0xbfffbeec --> 0x82f9774 (: mov eax,esi) 0016| 0xbfffbef0 --> 0x37400000 --> 0x2 0020| 0xbfffbef4 --> 0x80001000 ('A' ...) 0024| 0xbfffbef8 --> 0xe 0028| 0xbfffbefc --> 0x88dd0c0 --> 0x88dd0f8 --> 0x2 [------------------------------------------------------------------------------] Legend: code, data, rodata, value 0x0826e37a 122 zend_string *ret = (zend_string *)pemalloc(ZEND_MM_ALIGNED_SIZE(_ZSTR_STRUCT_SIZE(len)), persistent); gdb-peda$ ``` The size which is used as parameter in `_emalloc()` function is equal to `((oldlen * 2 + 0x14 ) & 0xfffffffc)`. Due to integer overflow, if `oldlen` is equal to 0x7fffffff, this size is 0x10. The new memory region is too small to store a large string! if we continue running, other memory region will be overwritten until SIGSEGV! ``` [----------------------------------registers-----------------------------------] EAX: 0x41 ('A') EBX: 0x199fa0 ECX: 0x37599fb0 ('A' ...) EDX: 0x3 ESI: 0x199fa1 EDI: 0xb7866050 --> 0x1 EBP: 0xbfffbf68 --> 0xbfffbfb8 --> 0xbfffc084 --> 0x0 ESP: 0xbfffbee0 --> 0x80001000 ('A' ...) EIP: 0x826eaf1 (: mov BYTE PTR [edi+ebx*1+0x10],al) EFLAGS: 0x10246 (carry PARITY adjust ZERO sign trap INTERRUPT direction overflow) [-------------------------------------code-------------------------------------] 0x826eae8 : mov ebx,DWORD PTR [ebp-0x38] 0x826eaeb : movzx eax,BYTE PTR [ecx] 0x826eaee : mov esi,DWORD PTR [ebp-0x30] => 0x826eaf1 : mov BYTE PTR [edi+ebx*1+0x10],al 0x826eaf5 : lea eax,[ebx+0x1] 0x826eaf8 : mov DWORD PTR [ebp-0x38],eax 0x826eafb : jmp 0x826e810 0x826eb00 : test BYTE PTR [ebp+0x14],0x2 [------------------------------------stack-------------------------------------] 0000| 0xbfffbee0 --> 0x80001000 ('A' ...) 0004| 0xbfffbee4 --> 0xb7ce07e9 (: pop ebx) 0008| 0xbfffbee8 --> 0xb7ce07f7 (: add ecx,0xc7809) 0012| 0xbfffbeec --> 0x82f9774 (: mov eax,esi) 0016| 0xbfffbef0 --> 0x37400000 --> 0x2 0020| 0xbfffbef4 --> 0x80001000 ('A' ...) 0024| 0xbfffbef8 --> 0xe 0028| 0xbfffbefc --> 0x88dd0c0 --> 0x88dd0f8 --> 0x2 [------------------------------------------------------------------------------] Legend: code, data, rodata, value Stopped reason: SIGSEGV 0x0826eaf1 in php_escape_html_entities_ex (old=0x37400010 'A' ..., oldlen=0x7fffffff, all=, all@entry=0x1, flags=0x0, hint_charset=0x88cce38 "", double_encode=double_encode@entry=0x1) at /root/fuzzer/PHP-7.1/ext/standard/html.c:1378 1378 ZSTR_VAL(replaced)[len++] = mbsequence[0]; gdb-peda$ ``` I can leak memory to bypass ASLR + DEP and control eip register to the arbitrary value. Finally, the overflow results as arbitrary code execution. The attached script which executes at local machine can leak library data and run `/bin/sh`. :)

Internet Bug Bounty: Heap overflow due to integer overflow in php_escape_html_entities_ex() function